As discussed here, even the so-called third eye, which merely provides light sensitivity to its owner such as the iguana, involves incredibly complex biochemistry. Whereas evolutionists have always envisioned a neat ladder-like pathway of increasing functionality in vision systems, even rudimentary vision such as the third eye reveals stunning complexity.

Introduction :-
A coelom is a fluid-filled body cavity found in most animals which is located in the mesoderm, derived from the splanchnic mesoderm, the middle germ layer only found in tiploblastic (three-layered) organisms. Simpler animals like Cniderians (ex- Jelly fish, Coral etc ) and sponges are diploblastic and monoblastic respectively, lacking a coelom.

It's rather fascinating that it is now thought that the last common ancestor of modern animals had all these advanced features, such as brain, nervous system , body segments, etc. Since many of these genes were lost in many lineages, that last common ancestor can even be said to be more advanced than many of it's descendants.